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Electronic Jetsam

Ocean rovers explore the depths and monitor the environment

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1:58pm, January 27, 2003
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Twice each day at more than 1,100 sites around the world, scientists simultaneously loft weather balloons to collect data about Earth's atmosphere. During each balloon's ascent, which lasts a couple of hours or so, instruments garner information about air temperature, humidity, barometric pressure, wind speed, and wind direction. Meteorologists feed all of these data into their computer models to forecast rain for next Tuesday or predict the weather for next year.

Oceanographers have long envied this wealth of data about the massive sea of air above them. If they could collect millions of daily measurements of currents, salinity, water temperature, chemical composition, plankton populations, and other features of the oceans, which cover 70 percent of Earth's surface, scientists could begin to model the vast, underwater world with finesse.

To alleviate their envy of meteorologists, oceanographers have been developing and deploying a variety of seafaring probes in ever-greater numbers. Like an array of remote sensory organs, these probes would report information back to the scientists on dry land. The probes can carry sonar systems for mapping the thickness of arctic ice, charting the seafloor around a roiling hydrothermal vent or searching for explosive mines in hostile waters. They also carry sensors to measure pressure, temperature, salinity, and other ocean traits. The probes pause intermittently to use their antennas to transmit data back to a home base via satellites or to receive revised instructions.

Such instrument-crammed ocean rovers will free scientists from sea duty, says Daniel J. Fornari, a senior scientist at the Woods Hole (Mass.) Oceanographic Institute (WHOI). In the past, he notes, oceanographers' observations have been "snapshots in time" limited by the availability of funding, personnel, ships, and time at sea. The coming era of drifters, gliders, and scientific torpedoes is poised to greatly enhance oceanographers' power to gather data, thereby transforming isolated snapshots into full-length feature films.

Bob, bob bobbin'

A global fleet of drifting probes that monitor conditions in the top layers of the ocean is giving ocean scientists a taste of what's to come.

Each float in this array, called Argo, looks like an oxygen cylinder. It's full of instruments and capped with a 70-centimeter-tall antenna. Sensors measure water temperature, electrical conductivity, and pressure, which tells the probe its depth in the ocean, says W. Brechner Owens of WHOI. With data from these sensors, scientists calculate the water's density and salinity, two of the driving forces for ocean currents.

Argo floats can be dropped into the ocean from research vessels, commercial ships, or even low-flying aircraft. Once deployed, the 26-kilogram probes at first drift with the currents about 2 kilometers below the surface. Then they pop back to the surface, collecting data en route.

To raise the capsule through the water, pumps shift about a cup of hydraulic oil from a reservoir in the cylinder to a small external bladder. As the bladder expands, the probe's overall volume increases slightly while its mass stays the same, so its density declines and the probe rises.

When the float reaches the surface, it beams data to researchers via satellites. That done, the pumps pull the hydraulic oil back inside the cylinder, and the probe again sinks and drifts for another 10 days before its next data-gathering ascent.

Argo probes, which are designed to last about 5 years, were first deployed in the year 2000, says Owens. More than 600 of them now bob through the world's oceans.

Oceanographers from a dozen or so nations plan to launch an armada of 3,000 Argo probes by 2006. In such a fleet, floats would be spaced, on average, 300 km apart.

Scientists are using Argo data to calibrate ocean measurements made remotely from Earth-orbiting satellites, as well as to directly inform ocean-current and climate models.

Sea wings

Not having motors, the Argo armada drifts along at the mercy of currents. To overcome this constraint, scientists have designed ocean gliders that are propelled by the same buoyancy-change technique used in the Argo probes.

Admittedly, such gliders are slow. They slip through the water at speeds of only around 1 knot—that's about 0.5 meter per second. On the other hand, their gravity-assisted trajectory doesn't require much electric power. Nevertheless, the lifetime of the batteries that power the oil movement within the glider limits its range.

That's why scientists such as WHOI oceanographer David M. Fratantoni are now developing a more energy-efficient way to pump oil back and forth. The new system will tap a source of energy readily available in most temperate-latitude oceans—the temperature difference between the warm water at the ocean's surface and the cold water thousands of meters below. The heart of the thermal engine is a tube filled with a wax that solidifies at about 10°C. The substance shrinks significantly when it solidifies.

The test bed for this novel engine is the Slocum glider, a craft that looks like a 2-m-long torpedo with slim wings. The wings ensure that the probe glides forward, plowing a sawtooth path through the ocean. The glider is named after Joshua Slocum, a New England captain who in 1898 became the first man to sail solo around the world.

Like the Argo float, the Slocum glider has an external bladder that changes volume as oil is pumped to and from an internal reservoir. When the glider is first placed in warm waters at the ocean's surface, some of that oil is pumped into the glider, increasing its overall density and causing it to sink. As the probe descends into colder water, the wax freezes and contracts, creating a vacuum that pulls yet more oil from the external bladder into the probe's interior. This action gives the pump battery a break.

When the glider reaches a depth of about 1,500 m, an onboard computer opens a valve so that compressed nitrogen can push oil back to the external bladder, increasing buoyancy. As the probe glides upward to warmer water, the wax melts and expands, ready for another cycle.

Fratantoni and his colleagues field-tested this propulsion system about 2 weeks ago in an ocean basin in the Bahamas that's more than 2 km deep. Preliminary analyses of the data downloaded from the probe after each of eight dives shows the propulsion system worked as expected. On a ninth dive, the glider encountered a problem unrelated to the propulsion system. Rough topography on steep terrain trapped the glider. "During field tests, you expect this sort of thing," says Fratantoni.

Scientists left the glider behind, but all is not lost. The probe, equipped with an acoustical beacon that can broadcast pings for the next 2 months awaits rescue in about 450 m of water.

Full speed ahead

To explore the ocean at speeds greater than 1 knot, investigators turn to autonomous underwater vehicles, or AUVs. Essentially battery-powered torpedoes for research, the probes can carry a host of sensors as they dive deep and travel several dozen kilometers.

AUVs are already routinely used by companies surveying the ocean bottom, for example, to identify optimal routes for pipelines or fiber-optic cables. In May 2001, an AUV surveying a pipeline route in the Gulf of Mexico discovered wreckage of the German World War II submarine U-166. The only sub sunk in the Gulf during the war, that U-boat went down on July 30, 1942, and now lies in about 1,500 m of water.

The Department of Defense, too, is finding AUVs useful. They can be sent into hostile areas to map the ocean bottom, look for mines, and assess environmental conditions. The U.S. Navy now has several Remote Environmental Monitoring Units—nicknamed REMUS—that can be programmed via laptop, dumped overboard by two sailors, and sent into harm's way in the stead of frogmen.

Now, more scientists are getting in on the AUV action. Consider the ALTEX (Atlantic Layer Tracking Experiment), which includes an AUV designed to monitor a layer of relatively warm water that flows from the Atlantic into the Arctic Ocean.

Oceanographers put the probe through its first arctic field trials in October 2001, says James G. Bellingham, director of engineering at the Monterey Bay Aquarium

Research Institute in Moss Landing, Calif. Last December at the American Geophysical Union's meeting in San Francisco, he and his colleagues described results of those tests.

Researchers deployed the ALTEX AUV from a Coast Guard icebreaker several times, says Bellingham. During the probe's first excursions under the ice, it ranged dozens of kilometers and traveled as deep as 500 m. All the while, it measured the temperature, salinity, and nitrate concentrations in the water and used sonar to determine the thickness of the icepack overhead.

The current version of the AUV is powered by metal-hydride batteries and has a range of about 50 km, but future models driven by longer-lasting fuel cells could cruise about 1,000 km. Other improvements on the drawing board include a mechanism by which the probe releases buoys that melt their way through surface ice and then broadcast data and information about the AUV's position back to the home base.

Other arctic AUVs could serve as roving seafloor seismometers. WHOI engineer Rob Sohn is dreaming up missions for the Autonomous Polar Geophysical Explorer, or APOGEE. In one scenario, the 2.5-m-long, 200-kg APOGEE would rest on the ocean floor, monitor and record earthquake activity for a certain period, and then make its way to the edge of the icepack or to a hole cut into the ice by a recovery team.

This AUV's modular design will let scientists outfit it any way they like, says Sohn.

One option might include sonar equipment to map the ocean bottom and the layers of sediment beneath; another might have the sensors tuned to look for the thermal and chemical anomalies associated with undersea hydrothermal vents.

Albert M. Bradley of WHOI says that AUVs are particularly suited to search for such vents. Compared with manned submersibles, AUVs can carry out longer, more systematic searches close to the ocean bottom. Once a probe's sensors detect signs of a hydrothermal plume, on-board computers could send the AUV into a three-dimensional flight plan to map the plume, take its temperature, measure its mineral content, and estimate its flow rate.

AUVs equipped with side-scan sonar equipment can map the ocean floor in great detail in a cost and time-efficient manner, agrees Fornari. In a single dive, for example, an AUV could map a square several kilometers on a side with enough resolution to pick out features just 1 m across. That economy makes AUVs ideal for the initial exploration of a certain area of seabed, for example, but it also enables scientists to make repeated visits and watch how features around midocean ridges or hydrothermal vents evolve.

In conjunction with other oceanographic equipment, AUVs can be particularly useful in so-called nested surveys, says Fornari. For instance, surface ships can map the ocean bottom on a broad scale and identify areas of interest, AUVs can then home in on those areas and map or photograph them in finer detail, and then scientists can descend in submersibles to investigate personally.

Or scientists could send forth fleets of AUVs to discover phenomena that merit further study. Permanent ocean observatories may be the home bases for such probes (SN: 12/7/02, p. 362: Ocean View), which could locate underwater electrical outlets, recharge batteries, download data, and perform self-diagnostic checks before receiving new marching orders and zipping back to duty.

Fornari says that these intelligent vehicles have the potential to "revolutionize" oceanography. Sohn agrees: "There's a place in oceanography for all types of vehicles, and there will be for decades to come."

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